1. Field of the Invention
The present invention relates to a small thin film movable element and a small thin film movable element array as well as a method of driving a small thin film movable element array for displacing a movable portion by an electrostatic force in according with a voltage applied to a movable electrode and a fixed electrode, which is preferably used in, for example, optical communication, an exposing apparatus, a projector, an optical switch, a scanner, an RF switch, an actuator, a multiplexor or the like.
2. Description of the Related Art
In recent years, by rapid progress of an MEMS technology (HEMS; Micro-Electro Mechanical Systems), there has been intensively carried out development of a small thin film movable element for electrically displacing/moving a small thin film of μm order. As the small thin film movable element, there are, for example, a digital micromirror device (DMD) for deflecting light by inclining a micromirror, an optical switch for switching an optical path and the like. In a field of an optical information processing, DMD is provided with a wide use of a projecting display, a video monitor, a graphic monitor, a television set and electrophotography printing and the like. Further, application of an optical switch is expected in optical communication, optical interconnection (a signal connection technology by light such as an intercoupling network by parallel computers), an optical information processing (information processing by optical operation) and the like.
A small thin film movable element generally includes a movable portion which is supported elastically displaceably and is displaced bi-directionally, and the movable portion mainly deals with switching operation. Therefore, a control of braking a movable portion becomes particularly important in carrying out excellent switching operation.
For example, a micromirror apparatus disclosed in JP-A-8-334709 is constructed by a constitution of applying a voltage to one electrode of a pair of drive electrodes, and rotating a movable portion having a mirror arranged between the electrodes by hinge connection by an electrostatic attractive force in accordance with a potential difference and an electrostatic capacitance between the movable portion and the drive electrode.
Further, according to a method of controlling to switch an optical switch disclosed in JP-A-2-7014, in an optical switch including a vibrating member displaced making a control voltage ON/OFF, and an element of reflecting or cutting propagated light by displacing the vibrating member at a front end of the vibrating member, before making the control voltage ON, a first preliminary voltage pulse shorter than a period of a natural vibration of the vibrating member is applied to the vibrating member, and after making the control voltage OFF, a second preliminary voltage pulse shorter than the period of the natural frequency of the vibrating member is applied to the vibrating member.
Generally, according to an optical switch, when a vibrating member is displaced by making a control voltage ON/OFF, a phenomenon referred to as chattering is brought about. The chattering is a phenomenon in which after making the control voltage ON or OFF, the vibrating member is not immediately changed by an amount of a displacement in correspondence with the control voltage but is displaced finally by the amount of the displacement in correspondence with the control voltage while carrying out a large attenuating vibration. Therefore, the reposes a problem that until the vibration is attenuated, and an optical output becomes a constant level, an optical path is not switched, and a speed of switching the optical switch is restricted. In contrast thereto, according to the method of controlling to switch the optical switch by the background art, by applying the preliminary voltage pluses shorter than the period of the natural vibration of the vibrating member before making the control voltage ON and after making the control voltage OFF, the chattering is controlled, and the speed of switching the optical switch is increased.
However, according to the micromirror apparatus disclosed in JP-A-8-334709, the voltage is applied to one of the contact electrodes, the electrostatic attractive force in accordance with the potential difference between the movable portion and the drive electrode and the electrostatic capacitance is generated and the movable portion is rotated. Therefore, as shown by FIG. 32A, immediately after the micromirror is transited to a contact position by applying a voltage Va and is grounded at the contact position, a vibration is produced by receiving a repulsive force from a contact member. Further, when the vibration is produced, in a case in which light constitutes a control medium, a fluctuation is brought about. Therefore, even a small thin film movable element having first rise by an electrostatic operation needs to await for converging the vibration, and as a result, there poses a problem of retarding an operational speed. Further, even when the micromirror is constituted by a noncontact structure in which the micromirror is not grounded to the contact position, as shown by FIG. 32B, by bringing about overshoot exceeding a desired rotational angle (converging position), time is required until converging the vibration. The vibration or the overshoot hampers high speed formation of a switching operation of the small thin film movable element.
Further, according to the method of controlling to switch the optical switch disclosed in JP-A-2-7014, before making the control voltage ON/OFF, the first preliminary voltage pulse, the second preliminary voltage pulse are applied to the vibrating member, the electrostatic force is operated in a single direction by one movable portion electrode and one fixed electrode, and vibration in driving the movable portion is restrained by a balance of forces of an elastic force and an inertia force of a movable support portion, the electrostatic force (potential difference) only in a forward direction operated in a transition direction of the movable portion is changed and therefore, there poses a problem that a vibration retraining effect is inconsiderable. Generally, in an optical switch for optical communication, different from DMD, much time is taken until converging free vibration since the optical switch is positioned by an arbitrary angle. Further, although high control accuracy is requested in order to reflect optical information of laser light or the like to a fiber on an emitting side to be incident thereon, the vibration of the movable portion (mirror portion) constitutes a cause of noise as the chattering. In this way, particularly in the case of the optical switch, an influence of the vibration is larger than that of DMD to pose a serious problem.
Further, in the small thin film movable element, there is an RF switch in which a movable portion includes a shortcircuit contact and an RF (high frequency) signal is connected and switched by opening/closing an input terminal and an output terminal in which the shortcircuit contact of the movable portion is provided at a board. Although in the RF switch, when a speed of operating the movable portion is several tens sec, the movable portion is not bounded, when the speed becomes about 5 μsec, the movable portion is bounded. Further, when the movable portion is bounded, chattering similar to the above-described is brought about to constitute a cause of bringing about a failure in operation. Therefore, there is requested an RF switch capable of carrying out smaller and firmer operation in high speed.